units: correct description of final.target

This was updated incorrectly in https://github.com/systemd/systemd/pull/20058/commits/4fd3fc66396026f81fd5b27746f2faf8a9a7b9ee.  As https://github.com/systemd/systemd/blob/main/man/systemd.special.xml decribes, this unit is about shutdown rather than boot.

(cherry picked from commit f127fed7)